The present invention relates generally to the field of microelectronics packaging and more specifically to packaging of microelectromechanical systems (MEMS) and integrated microelectromechanical systems (IMEMS) devices.
For current commercially packaged MEMS and IMEMS components, the steps of packaging and testing can account for at least 70% of the cost. The current low-yield of MEMS packaging is a “show-stopper” for the eventual success of MEMS. Conventional electronic packaging methods, although expensive, are not presently adequate to carry these designs to useful applications with acceptable yields and reliability.
Examples of MEMS and IMEMS devices include airbag accelerometers, microengines, optical switches, gyroscopic devices, sensors, imaging and microfluidic actuators. IMEMS devices can combine integrated circuits (IC's), such as CMOS or bipolar circuits, with the MEMS devices on a single substrate side-by-side, such as a multi-chip module (MCM). All of these devices use active elements (e.g., gears, hinges, levers, slides, mirrors, valves, etc.). These freestanding structures must be free to move, rotate, etc. during operation. Other types of microelectronics devices, such as microsensors, must be accessible and freely exposed to the environment during operation (e.g., for making chemical, pressure, or temperature measurements). Likewise, optical elements need to be accessible to the outside either directly or through a transparent window.
During conventional Surface Micro Machined (SMM) MEMS fabrication, silicon dioxide, silicate glass, or silicon nitride is used as a sacrificial material commonly used at the wafer scale to enable creation of complex, three-dimensional structural shapes from polycrystalline silicon (e.g., polysilicon or “poly”). The sacrificial layer (or layers) surrounds and covers the structural polysilicon MEMS elements, preventing them from moving freely during MEMS fabrication. At this stage, the MEMS elements are commonly referred to as being “unreleased”.
The next step is to “release” and make free the MEMS elements. Conventionally, this is done by etching and dissolving the sacrificial coating (typically SiO2) in a liquid solution of hydrofluoric acid (HF), hydrochloric acid (HCL), or a combination of both (e.g., a 50/50 mix). This wet etching step is conventionally done at the wafer scale in order to reduce processing costs. Other additions may improve stiction resistance. HF vapor release can also be used (the vapor arises from a mixture of HF and water, and, so is not completely dry.
After releasing the active elements, the MEMS devices are generally probed to test their functionality. Unfortunately, probed “good” MEMS devices are then lost in significant quantity due to damage during subsequent packaging steps. They can be easily damaged because they are unprotected (e.g. released). Subsequent processing steps can include sawing or cutting (e.g. dicing) the wafer into individual chips or device dies; attaching the device to the package (e.g. die attach), wirebonding or flip-chip mounting, solder bumping, direct metallization, pre-seal inspection; sealing of hermetic or dust protection lids; windowing; package sealing; plating; trim; marking; final test; shipping; storage; and installation. Potential risks to the delicate released MEMS elements include electrostatic effects, dust, moisture, contamination, handling stresses, and thermal effects. For example, ultrasonic bonding of wirebond joints can impart harmful vibrations to the fragile released MEMS elements. In the case of microsensors, the active sensing elements (e.g., chemically reactive films), may be damaged if exposed prematurely during the packaging process.
One solution to this problem is to keep the original sacrificial coating intact for as long as possible during fabrication. In one approach, the MEMS elements would be released (or microsensing elements made active by exposing them to the ambient environment) after all of the high-risk packaging steps have been completed. This is referred to generally as a “package first, release later” scheme.
Some problems exist with the use of wet acid etchants for releasing MEMS devices. There are safety and environmental disposal issues with using strong acids like HF and HCL. Also, the acid can attack and damage other features on the microelectronic device, including metal traces, lines, structures, films, bond pads, routing lines between bond pads, wirebonds, solder bumps, ring seals, bond interfaces, metallized layers, sensing materials, integrated circuits, such as CMOS or Bipolar structures and other semiconductor materials (e.g., standard photoresist protection used on CMOS or Bipolar chips may not provide sufficient protection from attack by acid etchants). Use of wet release etchants can also damage other fragile structures due to hydrodynamic forces, including alignment marks, logos, and test structures, wirebonds (e.g., due to wire sweep). Also, unprotected metallic structures (e.g., gold, aluminum, copper) on the device can be corroded by galvanic attack of the HF-bearing or HCL-bearing release solution.
Alternatively, a dry release etch may be performed by using a reactive plasma containing reactive oxygen, chlorine, or fluorine ions, or combinations thereof to remove the sacrificial layer(s). This eliminates the above-mentioned problems with using a liquid etchant, but is typically a slower process.
Released MEMS elements are often coated with a very thin (e.g., monolayers) anti-stiction, anti-adhesion, lubricating coating, (e.g. perfluoropolyether, hexamethyldisilazane, or perfluorodecanoic carboxylic acid, self-assembling monolayered films), and then dried.
In conventional microelectronics packaging, a common final packaging step (i.e., after mounting, wirebonding, soldering, for example) is to apply a protective, water-resistant coating of a parylene-type polymer or an epoxy encapsulant (glob) to all surfaces that might be exposed to moisture, etc. and to ruggedize the package. However, such relatively thick protective coatings cannot be applied to released MEMS elements (or to exposed microsensing elements), because the coating would prevent the MEMS elements from moving (or, for example, prevent diaphragms or membranes in a pressure-sensitive element from flexing).
Conventional photoresist materials have been used in packaged microelectronic devices for environmental protection. However, because of their high viscosity (from 1000–17,000 cts), delicate wirebonds can be damaged due to excessive wire sweep caused by high viscous shear forces applied during deposition of the photoresist (e.g., during spin coating).
Also, in conventional fabrication of MEMS devices that use movable micromirrors (e.g., for digital light projectors, DLP's), a common final packaging step is to deposit a flash of gold (possibly with a Ti or Cr adhesion layer (Cr survives release etch)) onto the mirror surfaces to give them the desired reflectance or other optical properties (but not being so thick as to interfere with their motion). However, this step requires that the surrounding areas containing electrically conductive lines, pads, wires, etc. are masked off to prevent the conductive gold film from short-circuiting them.
Also, problems can exist from excessive electromagnetic cross-talk interference between neighboring conductors (signal lines, power lines, wirebonds, etc.) because they are electromagnetically unshielded from each other. A thin, metallic overcoat could act as an electromagnetic shield, but this would also short-circuit the device since the exposed metallic conductors are not electrically insulated. This would require that each individual conductor (e.g., gold wirebond) be coated with an insulating layer (which is not presently done, because application of the insulating layer would coat the released MEMS elements and prevent them from functioning).
The parent applications to the present invention describe certain disadvantages of prior art techniques and disclose methods for protecting MEMS and IMEMS particularly during packaging. The present invention provides for protecting such devices through a release etch or etches.
What is needed, therefore, is a method for protecting not only MEMS elements and microsensor elements in the “active” area of a device, but also integrated circuits, other structures on the die, contact/bond pads, wirebonds, other interconnects, solders, packaging materials, adhesives, etc. in the “passive” area from physical, chemical, or electrical damage during release or other activation procedures in a “package first, release later” scheme, without having to use a thick, bulk encapsulant (such as plastic injection molding).